The present invention relates generally to improvements in image focussing mechanism, and it relates more particularly to an improved mechanism for determining the focus of an image by an objective lens on a predetermined focal plane, and to the combination therewith of a through the lens light measuring system.
A mechanism has heretofore been proposed for detecting the focussed condition of an image formed by an objective lens on a focal plane in which an image is scanned to obtain a video signal and the video signal is then differentiated or passed through a high-pass filter to derive a high frequency component indicative of the focus based on the phenomenon that a sharp image contrast results when proper focussing is effected. To this end, a scanner having a slit or pin hole is oscillated or rotated in a direction parallel to the image plane in the vicinity of the image focussing plane of a lens, and a video signal is thus obtained or produced by a photoelectric element placed in the rear of the scanning slit or pin hole. However, the aforesaid type of mechanism possesses important drawbacks in that there is provided as a scanner a plate having a sufficient area to shield the photoelectric element throughout the course of its oscillation or rotation, the size of the device including the drive source is large and bulky, with a resulting large energy consumption and the mechanism otherwise leaves much to be desired.